Speaker
Description
At elevated temperatures polycrystalline materials creep. As they creep, cavities can nucleate on grain boundaries, grow and coalesce to form microcracks, which then link causing failure. Some of these stages are well understood, but despite over 50 years of extensive research there is no generally accepted model of cavity nucleation, with empirical models based on experimental observations [1] not being consistent with classical physically based models [2]. Classical models effectively predict a threshold stress for nucleation above which cavities nucleate spontaneously, which, unless unrealistic low interfacial energies are assumed, is much higher than typical stress levels at which cavities are observed to nucleate. In practice cavities nucleate continuously throughout life, as captured by empirical models. Ad hoc attempts have been made to bridge the empirical and classical models, but with little success.
One problem with the classical model is that it does not fully take into account the effect of the presence of other defects, such as dislocations. Dislocation structures that develop during creep perform two roles; they influence the stress state local to the nucleation site; and they change the energetics of the nucleation process, with dislocations providing vacancies for the nucleating cavity. We explore these two contributions.
In this work, we reformulate the classical nucleation theory. We analyse GNDs (geometrically necessary dislocations) at precipitate/matrix interfaces, and the rotation of low energy tilt grain boundaries. The new formulation results in the introduction of an additional contribution to the energy barrier, with the dimensions of stress, σ_d, which has the effect of reducing the critical energy for nucleation, ∆G_c, the critical size of the nucleated pore and the stress for nucleation.
[1] B.F. Dyson et al. Proc R Soc London Ser A. 349 (1976) 245–259
[2] R. Raj, M. Ashby, Acta Metall. 23 (1975) 653–666
| Speaker Country | United Kingdom |
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